I will never buy an Unidentified item from a stranger. It is too easy to test the mods (health being the easiest). The only reason to buy one is for unlocking mods for pvp. I never understand why ppl sell unid sheilds because it is very easy to test its stats and you cant get unlocks from them anyway!!
A skin is the look of your items, not sure about pics there must be some in here. Try a search.
As is often the case, there's a long thread about this on the site.
But, yes, many people do test unidentified weapons before selling. With a little experience it is easy to discover most mods (other than mod that have little effect on damage such as sundering).